Wind

Wind is one of the oldest of man’s energy resources initially powering ships. Before the steam engine; windmills together with waterwheels were the only means of doing mechanical work that did not involve muscle power.

For example Sydney once had 19 large windmills; gradually replaced by steam engines burning wood and coal.

Wind has remained important for water pumping on rural properties; with tens of thousands of locally manufactured wind pumps still installed. Small scale electricity generation using wind-power is a widespread alternative to photovoltaic solar in rural locations where grid connection is impractical and on boats (particularly yachts).

Comet Windmills at Macksville NSW have been manufacturers of wind-pumps since 1879. Small farm, boating and local wind machines sometimes derive up to 50kW (peak) energy from the wind (typically a lot less).

By comparison industrial scale wind turbines are very large machines with peak energies typically in the MW range. Commercial wind farms typically comprise between 50 and 100 2MW wind turbines. Wind is now the most competitive renewable electricity generation technology after hydro-electricity.

Advantages of Wind Energy

When compared with all other current electricity generation technologies, wind energy has a very low carbon footprint. When large modern turbines are located in a well chosen wind province, so that the capacity factor yields a commercial return, only run-of-river, hydro-electricity and nuclear electricity produce less carbon dioxide per kilowatt hour (kWh).

Wind Technologies

The past 20 years has seen a maturing of wind technology with a move to larger, more complex wind turbines, standardising on the now familiar three blade up-wind design for its vibration free, reliability and efficient and relatively silent operation.

Until recently Denmark Germany and the US dominated wind turbine manufacture. But companies in India and China have recently entered the market by acquiring the rights to existing technology under licence or by direct acquisition. The industry is still relatively volatile with many earlier entrants, high levels of company failure and corresponding mergers and acquisitions. A few larger companies are now beginning to dominate.

The Indian company Suzlon claims to have most of the Australian market. In the region China’s Goldwind is growing rapidly and recently acquired Germany’s Vensys.

By installed capacity (not current sales) the top ten wind turbine manufacturers two years ago were:

Rank

Company

Location

Installed

1

Vestas

Denmark

35,000 MW

2

Enercon

Germany

19,000 MW

3

Gamesa

Spain

16,000 MW

4

GE Energy

Germany / United States

15,000 MW

5

Siemens

Denmark / Germany

8,800 MW

6

Suzlon

India

6,000 MW

7

Nordex

Germany

5,400 MW

8

Acciona

Spain

4,300 MW

9

REpower

Germany

3,000 MW

10

Goldwind

China

2,889 MW

China is the world’s fastest growing wind market with over 105% growth for the last four years and projected to overtake the US (35.2GW) as the largest user this year. Australia has less than a twentieth of that installed capacity and a relatively lacklustre growth rate.

Larger machines offer economies of scale and provide better access to the wind resource Increasing the swept diameter substantially increases annual electricity output, partly because the swept area is larger (by the square of the radius) and partly because the tower height increases correspondingly; and wind speed and quality increase with distance from the ground.

But there are technical limits to this up-scaling, including materials technology, centrifugal forces and aerodynamics. The blade must be very strong and relatively light. Blades are typically made from glass reinforced plastic (GRP) and incorporate lightning protection measures. The maximum tip speed of a turbine blade is limited by turbulence and noise to around 300 km/hr (and ultimately by the speed of sound, about four times this speed).

The 2.1 MW machines at Capital Wind Farm recently completed in NSW are close to the present commercially optimum size (with blades 41 to 45 metres long rotating at 17.7 RPM; a tip speed of 300 km/hr). The turbine is supported on an 80m tall steel tower 4.3m in diameter at the base, tapering to 2.3m at the top.

Source: Capitol Wind Farm Publicity Shot

Although machines over three times larger are in operation they do not appear to offer substantial commercial advantage (through a cost reduction per MWh generated[18]).

Advances in electronic monitoring and controls, blade design, mechanical and electrical design have also contributed to a drop in the capital cost per kWh of electricity generated. Capital Wind Farm averaged AUD$3.13 million per 2.1 MW wind turbine installed. The Indian turbine manufacturer Suzlon Energy Limited was the turnkey contractor responsible for the Engineering, Procurement & Construction (EPC) of the entire project as well as ongoing service and maintenance of the wind farm.

This comprised[19]:

Design and manufacture of the wind turbines; Detailed in-house wind turbine micro-siting; Grid dynamic studies; Design, construction and maintenance of more than 30km of new access roads; Design and construction of footings and hardstands for each tower; Design, fabrication and installation of steel turbine towers; Shipping, installation and commissioning of the turbines; Design and installation of electrical feeder systems both below and above ground; Linking the turbines to the substation; Design and installation of a new 330kV substation, including two 330/33kV transformers; 240km of high tension cables for rock-anchor footings; 11,300T of steel for towers; 60km of underground cable; 10km of overhead cable.

Large wind generators have a projected working life of 20 years but require ongoing maintenance. Towers and blades are subjected to continuous flexing and vibration, in turn transmitted to bearings and the gearbox. Recently Suzlon have had to replace a number of blades overseas after only a few years operation due to inadequate manufacturing or design specifications. Electrical equipment and electronic controls may also require regular maintenance and replacement.

By comparison wind turbines are higher maintenance than hydro generators but relatively low maintenance compared to gas turbines; or coal plant; and most solar thermal plant designs; with a similar generating capacity.

Larger wind farms (clustering many machines) are more economical than smaller ones due to transaction costs, operations and maintenance costs that can be spread over more kilowatt-hours with a larger project. The American Wind Energy Association attributes cost savings of up to 40% for a project of 51MW (like the Capital Wind Farm), compared to a small farm of one or two machines[20].

There is considerable mechanical and electrical technology associated with a large modern wind turbine. Wind changes speed and direction. Like all other horizontal axis machines the whole nacelle needs to turn on the tower (yaw) to face the wind; requiring an additional drive subsystem. The turbine needs to be slowed or stopped to allow any substantial change of direction or serious damage to the structure is likely to result from twisting due to angular momentum (the gyroscopic effect).

The wind energy depends on air speed and mass (density). Both of these vary; air density declines with both increasing air temperature and increasing humidity[21]. Dust and rain also alter blade aerodynamic performance. Modern machines can be made to extract more wind energy from a given resource by adjusting the blade pitch to an optimum for the prevailing temperature, humidity and wind speed.

In large AC machines used to supply power to the grid, the rotational speed of the alternator is very close to constant while connected, the alternator being synchronised to the frequency of the grid. Any attempt to drive the shaft faster results in an advancing waveform and additional energy being absorbed by the grid. This correspondingly increases the current generated by the alternator (the voltage remaining constant).

A gear box between the turbine hub and the alternator holds the operational rotation of the turbine blades in constant relationship to that of the alternator. As indicated above in large machines this is dictated by the practical tip speed of the turbine blade. As a typical three phase 50 cycle alternator has a rotational speed of 1500 rpm a gearbox is required to increase (in this case) 17.7 rpm to 1500 rpm. This can’t be done in a single step so a multistage gearbox capable of handling the turbine’s peak power (>3,000 horsepower) is required.

The Suzlon S88, used in the Capital Wind Farm, has a three stage gearbox (one planetary stage & two helical). These gearboxes and associated bearings are amongst the largest manufactured today. All gearboxes have a small efficiency loss of around 1% per stage and this loss appears as heat. 3% of 2MW is around 60kW (30 electric room heaters at full blast) and this needs to be dissipated through an oil cooler and additional equipment in hot climates.

A transformer in each structure converts the ~690V AC @ 50 Hz generated by each turbine to 33kV AC for transmission to the grid (in NSW). As each turbine gains initial operational speed it needs to be locked into phase with the grid before its power can be delivered, typically using thyristor (solid state electronic) cut-in. Once it is powered up the thyristors are typically bypassed to save losses. When wind speed is low the alternator, if still electrically connected, will act as a motor and attempt to drive the blades (a giant fan) consuming power from the grid. Thyristor controls disconnect the alternator in these conditions (and the blade rotation may be stopped using a brake).

In high wind the turbine power can potentially rise until the alternator or drive train exceeds its rated maximum power. Prior to this point the turbine must be shutdown. If, during high wind the alternator exceeds its ability to absorb the power and fails; or if it is suddenly disconnected from the load (for example if the grid transmission line breakers trip when hit by lightening); or the drive train breaks; the turbine rotation will rapidly accelerate (runaway), with exponentially growing total energy.

One or more of these faults may cause the alternator or other components to catch fire. The energy and torque developed in a runaway turbine quickly exceeds the capability of the main shaft to stop it even if the mechanical brake could stop the shaft rotation. If allowed to continue the whole machine will be destroyed catastrophically (due to a blade or bearing failure; leading to an out-of-balance condition; nacelle detachment; blade contact with the tower; and/or tower failure).

Watch a spectacular runaway and deconstruction; there are related links to images of wind turbines on fire:

No responsibility is accepted for linked third party video or media content - see Terms of Use and Copyright.You follow the YouTube links displayed at the conclusion of this video clip at your own risk.

All large wind turbines have multiple safety devices to prevent such a runaway failure.

Some turbines have electrically or hydraulically driven tip brakes to stop the rotation in high wind, in others the equipment used to set the pitch of the blades is used to stop the turbine by feathering the blades. All large machines have at least one mechanical brake and brakes are held open by the proper functioning of the machine so that they ‘fail-safe’. Wind speed, direction and air density are continuously monitored at each machine. Any mismatch between the machine performance and the wind measurements immediately triggers the fail-safe equipment.

Like fluky wind that changes direction, gusty wind that repeatedly pushes a turbine over its rated wind speed limit will cause frequent shutdowns and restarts, seriously impacting the capacity factor.

There are alternatives to the now ‘run of the mill’ 2MW AC design. For example the very large Enercon machines use an annular low-speed synchronous generator that requires no gearbox. The turbine speed is allowed to vary to match wind conditions using variable pitch control on the blades. The output voltage and frequency vary with wind speed and are first rectified to DC then inverted for output to the grid.

Elaborate solid state power electronics and control substitute for the mechanical systems of the more typical AC design. Other manufacturers have alternative alternator configurations including those with dual windings that allow multiple turbine speeds for different wind conditions.

All of these control systems need to be automated using computer technology and to be remotely connected to a central control system. The design of these systems and the related control software is fundamental to the efficiency and safety of the machine and the resulting capacity factor. It is the closely guarded intellectual property of each manufacturer.

In addition to the conventional upwind three blade turbine used commercially, there is a vast array of alternative turbine designs, including vertical designs that operate independently of wind direction. These are commonplace in the ‘small farm generator’ and ‘battery charging’ market and there are many speculative designs that involve scaled-up versions. With the exception of two blade designs, to date, none of these alternative turbine designs has been taken up commercially by large wind farms for significant supply to the power grid.

Disadvantages of Wind

Large scale wind generated electricity is constrained by:

The availability of practical sites, with good quality wind resources, within an economic distance of electricity demand (transmission costs).

The relatively high cost per kWh, compared to conventional energy sources.

Its unpredictable, intermittent nature and the high cost of electricity storage.

The economic viability of wind power drops quickly as the quality of the wind resource declines. Most new well placed wind farms achieve a capacity factor of over 30% and this provides a good commercial return at about twice the thermal energy cost. But some older farms do not achieve 20%. Denmark, the largest (by proportion) exploiter of wind in the world and leader in wind technology, achieved a global capacity factor of 26.2% in 2007[16]. Several sources state that the average capacity factor of wind in Germany in 2003 was only 15% and that the average capacity factor in the UK was 24% despite some very high rating wind farms in Scotland and off-shore[17].

Many EEC countries implemented cap-and-trade market mechanisms and other incentives for renewable energy and were early wind adopters, with Germany having the largest installed capacity in the world until 2007 (on the basis of carbon cap-and-trade and feed-in tariff incentives). It is evident that in the rush to ‘make a euro’ off the back of these schemes, many locations were ill chosen. In addition earlier turbines were less efficient at exploiting poor wind conditions. This gave incentive for German firms to lead the technological improvements now evident and, as will be seen later, for their becoming acquisition targets for the new wind ‘super powers’: India and China.

A similar situation prevailed in respect of solar energy incentives with Germany a huge early adopter of initially technically unproven technologies.

Various attempts at energy storage to smooth out fluctuations in the wind have been suggested including batteries and hot salt (as used by large thermal solar plant) but these add so much to the capital cost that wind producers prefer to suffer the possibility that energy markets will not accept their production when consumer demand is low and wind is strong.

This, in effect, sets a top limit on the proportion of wind power that it is economic to attach to a grid. This limit is discussed later. Like solar, wind power (without storage) is dependent on other generators to meet the consumer demand when there is no wind (or sun).

The Australian Wind Experience – An Example

In NSW good wind provinces wind are not as plentiful as in the South and wind presently provides less than 0.2% of the State’s electricity.

There are now ten wind generators in NSW having a combined capacity of 149MW (0.15 GW). A further 33 (2.9 GW) are proposed. Total installed wind capacity within the National Energy Market (NEM, encompassing the Eastern states and SA) is presently 1.7 GW, with a further 650 MW is under construction.

The largest wind farm in NSW is the Capital Wind Farm near Bungendore that comprises 67 2.1MW wind turbines[15] with a total installed capacity of 140.7 MW and an annual (projected) production of 450,000 MWh/yr.

A wind farm is a substantial commercial undertaking. The publicly announced cost of the Capital Wind Farm project was $210 million. This scale of undertaking requires an assurance of a sound return to investors.

The location of a wind farm and proper initial mapping of the wind resource and subsequent micrositing of each turbine is critical to achieving an economic energy cost and justifying the claimed environmental credentials of wind (a low carbon footprint).

The economics and capacity factor are very sensitive to the quality of the wind resource. This needs to be as constant as possible and not fluky. Uninhabited ridge linesare usually preferred. For example the Capital Wind Farm is anticipating 450,000 MWh/yr (from 140.7 MW installed); a very optimistic 36.5% capacity factor.

Source: Capitol Wind Farm Publicity Shot

The Capital Wind Farm site covers more than 35 square km on the Hammonds, Ellenden and Groses Hill ridgelines 60km North East of Canberra. The owner is Renewable Power Ventures Pty Ltd - a subsidiary of Infigen Energy Ltd.

For reasons of transmission line cost and grid losses under the present value of RECs, the practical maximum distance of a wind farm from the present NSW 330kV grid is limited to few tens of kilometres. As solar electricity is presently more expensive to collect, the economics of large scale solar are even more adversely affected by distance to the grid.

For example, Capital Wind Farm connects to the grid via a 10 km long 33kV transmission line. This is stepped up to 330kV at the grid connecting sub-station. Nevertheless transmission infrastructure contributed significantly to the cost of the project.

As already indicated in the introduction wind and solar energy costs are very dependent on the total up-front construction costs and the method of financing these. Minimising the initial investment relative to the expected energy yield is critical to commercial viability. But revenue is dependent on energy delivered and as indicated above the pre-REC, NEM price is influenced by transmission costs and the point on the grid into which electricity generated is injected.

Even with the present substantial and probably increasing REC cross-subsidies from electricity consumers, transmission factors limit the economic distance an exploitable wind or solar resource can be from the main electricity grid and electricity consumers.

Many of the remaining wind resources that combine: a good wind province; proximity to the grid and electricity markets; and a sufficiently large precinct for an economic grouping; are not presently available for property access, planning or environmental reasons. There is also a growing, largely rural, anti-wind protest movement, both in Australia and overseas.

The intermittent nature of wind power requires transmission capacity to handle its maximum output. This requires a plant around three to five times larger than is justified by the average load. The up-front cost of a long transmission line adds significantly to the resulting net energy cost. There remain very good, continuous but remote wind provinces in the South of the continent that might support a very large wind farm, combined with limited storage and fast open cycle gas generation to smooth wind fluctuations and HVDC transmission back to the grid. But it is unlikely that any wind farm in NSW would justify the additional capital cost involved.

Wind is seldom a good match for the customers’ consumption requirements. These fluctuate significantly but in a predictable way. Denmark has found that at about 20% wind (average contribution) there are periods when the peak energy available exceeds the customer demands. Although they are attached to the European grid and can export these peaks, the price received is likely to be zero. These large fluctuations also have serious grid stability and management implications. They are particularly adverse for remote communities with insufficient facility to dispose of generation peaks at times of low demand or draw on other resources when demand is high.

In Eastern Australia the AEMO will cut wind farms off the grid when power generated exceeds the capacity of consumers to accept it. If, with more entrants, farms are increasingly unable to sell power at times of optimum wind, the commercial viability of additional (and existing) wind capacity will be seriously degraded. When turbines are turned off for long periods the benefits of carbon saving are lost as stationary wind turbines have an infinitely large carbon footprint. The carbon expenditure per kWh then exceeds that of coal.

South Australian and Tasmanian farms are approaching this limit, and are already selling peak electricity into Victoria. SA currently has just over 1,000 MW of installed wind capacity to which needs to be added a minimum base load from conventional power stations.

In addition, there are 30 wind proposals in SA totalling up to an additional 3,000 MW, including a $1.0 billion 600MW (300 turbine) farm located between Beachport and Mt Benson. If most of these eventuate, the lower production curves in the above diagram will expand vertically to completely overlap the demand curve and SA will have peak wind capacity well in excess of the local ability to consume the power available (even at peak summer demand) during optimum wind conditions. As a consequence a large number of turbines will need to be shut down as the wind energy rises. The overall wind capacity factor in SA will then fall steeply (and the carbon footprint of the wind turbines will rise).

At the same time SA must maintain conventional capacity sufficient to cover periods when wind is not blowing. These factors will both limit the further deployment of wind power in SA and require additional infrastructure to import and export electricity to Victoria and the NEM. This inevitably involves significant energy losses, additional electricity production and higher electricity prices across the NEM.

It is partly for distribution reasons that the adoption of wind power has been relatively slow in Australia. Wind presently contributes 0.9% of Australia’s electricity and if all the presently projected developments eventuate, this will just double. It remains to be seen what future projects the projected escalation in the price of REC’s and an ETS will encourage.

For example if the REC price is forced up significantly by a shortfall in economic renewable resources, the owners of existing wind farms will make very substantial ‘windfall profits’ (for no effort on their behalf) while at the same time very little additional renewable energy will result (due to the very conditions that forced the price up).

Thus there will be pressure to devalue the REC (as happened for rooftop solar), for example by offering additional RECs for transmission infrastructure or to relax the mandatory targets in favour of voluntary targets.

Again the total cost would be borne not by the companies involved (that may be very profitable) but by the electricity consumer.

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